The present invention concerns an adaptable filtering device and method for signals received by an active sonar, mounted on a mobile carrier, for rejecting reverberation signals.
It is known that a sonar-type apparatus on a carrier vehicle in motion (boat, submarine, torpedo, etc.) detects and localizes other vehicles or objects by emitting ultrasonic waves throughout the surrounding under water space(hereinafter called insonified space) and by observing the echoes received as to their distance and their direction. It is also well known that the ultrasonic energy thus transmitted in the underwater medium is reverberated by particles, bubbles and other discontinuities present in the medium or on its surface.
This reverberation interferes with the reception of the echoes since it tends to mask them. One means used to diminish the reverberation is to form at the reception, very directive angular paths, but this means is limited by the dimensions of the antenna.
The carrier vehicle being in motion, the frequency of the emitted signal sent back by an immobile target is shifted by the Doppler effect, peculiar to the carrier; this is the case for the reverberation, in which heterogeneities of the medium behave like fixed targets. When the target is in motion, which is the most frequent case, the frequency of the signal sent back by the target is shifted both by the Doppler effect due to movement the carrier and the target. It follows from this that the frequency of the signal corresponding to the reverberation and that corresponding to the target are different. It is known to use this difference of received frequencies for separating the signals from the echoes of the reverberation.
Thus, in the sonar-type electro-acoustic apparatuses, a band rejection filter is used in order to attenuate the reverberation. This filter generally has a fixed band width and a central frequency that varies as a function of the frequency shift caused by the Doppler effect due to carrier movement, i.e. as a function of the speed of this carrier. A filtering device of this type, applied to a torpedo sonar, is described in U.S. Pat. No. 3,723,954.
In this device the bandwidth of the rejection filter is calculated as a function of an average speed. Then, this band width varies with speed, the observation direction, i.e. the direction of the path and, as will be seen, with the width of the directivity lobe of this path.